In recent years, highly heat-resistant thermoplastics are increasingly demanded in a wide variety of fields including parts such as those for electronic instruments and motor vehicles.
PAS represented by poly(p-phenylenesulfide) (hereinafter abbreviated as "PPS") attracts attention as an engineering plastic having good heat resistance. However, it has been difficult to stably provide high-molecular weight linear PAS by conventional production processes. Consequently, this situation gives rise to the problem of difficulty in obtaining, in particular, fibers and films for which high strength is required or molded articles for which high impact strength is required.
As a typical process for producing PAS, there is a process wherein a dihalo-aromatic compound is reacted with sodium sulfide in an organic amide solvent such as N-methylpyrrolidone (U.S. Pat. No. 3,354,129). However, the PAS produced by this process has low molecular weight and melt viscosity, and it is difficult to fabricate it into a film, sheet or fibers.
Under such a state of the art, various proposals have been made to improve the process as mentioned above in order to obtain PAS with higher polymerization degree.
For example, it has been proposed to use an alkali metal carboxylate as a polymerization aid in the above reaction system (U.S. Pat. No. 3,919,177). According to this process, it is necessary to use the polymerization aid in an amount substantially equimolar to the alkali metal sulfide. Further, for obtaining PAS with higher polymerization degree, an expensive lithium acetate or sodium benzoate is required for use. This requirement results in increased production cost of PAS, resulting in commercial disadvantage. Also, according to this process, a large amount of organic acid may be entrained in the disposed waste water during recovery of PAS, whereby problems in pollution may be caused. For prevention of such problems, enormous costs are undoubtedly necessary.
It has also been proposed to use a trivalent or higher polyhalo-aromatic compound as a crosslinking agent or a branching agent (U.S. Pat. No. 4,116,947, etc.). According to this process, it is possible to obtain a high-melt viscosity PAS. However, since this PAS is not a linear polymer, but a polymer crosslinked or branched to a high degree, it has poor fiber-forming property, and it is difficult to form it into films or fibers. Also, even if molded or formed articles could be obtained, there is still the problem of their being mechanically extremely fragile because their molecular chains are basically short.
The present inventors previously carried out an investigation with respect to a process for cheaply producing a high-molecular weight PAS without use of a polymerization aid such as an alkali metal carboxylate, and studied in detail on the mechanism of polymerization reaction of an alkali metal sulfide and a dihalo-aromatic compound. As a result, it was found that a linear PAS of a high molecular weight with a melt viscosity of 1,000 poise or higher as measured at 310.degree. C. and a shear rate of 200 sec.sup.-1 can be readily produced without use of an aid by using a two-step polymerization process and changing, in particular, the water content and polymerization temperature among various polymerization conditions to a significant extent between preliminary and final stages of polymerization (U.S. Pat. No. 4,645,826).
However, in conventional polymerization processes including this two-step process, when a reactor made of a general-purpose material such as stainless steel was used as a polymerization reactor, it was necessary to strictly control the water content in the initial stage of the polymerization within a relatively narrow range of, for example, 0.5-2.4 moles per mole of an alkali metal sulfide charged in order to obtain a high-viscosity PAS and avoid the decomposition of the PAS formed. On the other hand, alkali metal sulfides commercially available as industrial materials are generally salts containing a large amount of water, such as trihydrates, pentahydrates or nonahydrates. It was necessary to dehydrate and remove an excess amount of water from the salt t be used prior to initiation of the polymerization reaction to strictly control the water content of the salt.
The present inventors made a further investigation with a view toward omitting or reducing the energy, equipment, time, process complexity, etc. which are required for such a dehydration process. As a result, it was found that when a reactor at least a portion of which, said portion being brought into contact with the reaction mixture, is made of titanium, is used, a high-melt viscosity PAS can be easily obtained even if the water content is relatively high upon the preliminary polymerization (U.S. Pat. No. 4,745,167). However, since the polymerization system became somewhat unstable though the dehydration process could be omitted, it was necessary to conduct the preliminary polymerization for a long period of time at a low temperature in order to avoid an decomposition reactions during the polymerization.
Alternatively, it has been proposed to cause the oxide or hydroxide of an alkaline earth metal to exist in the reaction system in the production process of PAS (Japanese Patent Application Laid-Open No. 51034/1 986 and U.S. Pat. No. 3,869,433). However, these processes require the addition of the hydroxide or the like in a relatively large amount. Therefore, when these processes were applied to the two-step polymerization process for producing particles of a high-molecular weight PAS, it was difficult to remove fully the remaining alkaline earth metal ion or the like from the PAS recovered after completion of the polymerization reaction, resulting in an adverse influence on physical properties such as clarity. In addition, in these known processes, the resulting polymers indeed tend to adhere to the wall surface of a reactor and a stirrer. They hence encounter difficulties in processing and in providing polymer particles with uniform particle size.